1. Field of the Invention
The present invention relates to a head unit having a magnetic head of a type which applies a load (a pressure to be applied) to a disc-shape recording medium when a signal is recorded or reproduced.
2. Related Background Art
An overwriting-type magneto-optical disk using modulation of a magnetic field must be subjected to a process that the ferromagnetic field is modulated with high-frequency waves. Therefore, a magnetic head of a recording/reproducing apparatus (a hard disk drive unit) must be disposed adjacent to the signal recording surface of the magneto-optical disk.
As a method of disposing the magnetic head adjacent to the signal recording surface of the magneto-optical disk, a method arranged as shown in FIG. 1 has widely been used. The method is arranged in such a manner that a magnetic head 21 is joined to a floating-type slider 22. The slider 22 is joined to the leading end of a suspension 23 constructed of a leaf spring. Thus, the slider 22 is pressed against the signal recording surface 1a of the magneto-optical disk 1 by the spring force of the suspension 23.
When the magneto-optical disk 1 is rotated at high speed, air is introduced into a space between the magneto-optical disk 1 and the slider 22, as shown in FIG. 2. Thus, an air film layer 41 is formed which causes the slider 22 to float from the magneto-optical disk 1 at a predetermined height h.
The reason why the magnetic head 21 is formed into the floating-type structure also lies in that the data-transmission rate must be raised. To raise the data-transmission rate, the recording density in the direction along the track lines of the disk must be raised. Moreover, the rotational speed of the disk must be raised to raise the data reading/writing speed in the direction of the track lines.
For example, a magneto-optical recording and reproducing apparatus of a type which is known as a minidisk system includes a head unit having a magnetic head which slides on the surface of a disk while applying a relatively low pressure to the disk. The reason for this is that the disk is rotated at relatively low speed. If a disk for use in a computer system or a multimedia system requiring higher recording density is rotated at high speed, there is apprehension that the surface of the disk is scratched or the load which is imposed during the sliding process is enlarged if the sliding-type magnetic head is employed.
If a disk having the sliding surface, the flatness of which is unsatisfactory, is rotated at high speed, a complete sliding state cannot be realized. In this case, the slider having the magnetic head mounted thereon jumps on the surface of the disk. It leads to a fact that a constant distance cannot be maintained from the recording surface of the disk to the magnetic head. As a result, the recording operation cannot satisfactorily be performed.
The above-mentioned problem experienced with the sliding-type magnetic head can be solved by the floating-type magnetic head. Even if the rotational speed of the disk is raised, the surface of the disk cannot be scratched or the load which is imposed during the sliding process cannot be enlarged. Moreover, a constant distance can be maintained from the recording surface (the recording layer) even if the flatness of the surface of the disk is unsatisfactory. Therefore, the ferromagnetic recording and reproducing systems requiring a high density recording operation usually include the floating-type magnetic heads.
The recording and reproducing apparatus including the floating-type magnetic head and structured as shown in FIG. 1 is required to stabilize the attitude of a slider 22 during the period in which the slider 22 is allowed to float. To achieve this, the slider 22 is pressed against a magneto-optical disk 1 under a predetermined pressure by the spring force of a suspension 23. Therefore, a load imposed by the suspension 23 and the like is applied to the magneto-optical disk 1 because of the reaction of the slider 22 during the period in which the slider 22 is allowed to float. Therefore, force for inclining (deflecting) the magneto-optical disk 1 acts on the magneto-optical disk 1.
If the magneto-optical disk 1 has a small radius and a large thickness, the magneto-optical disk 1 is not inclined (deflected) even if a load is applied. Even if the magneto-optical disk 1 is inclined, the amount of the inclination is negligible because the amount of the inclination is too small to adversely affect the performance for recording a signal.
In recent years, the requirement for raising the recording density results in the thickness of the disk being reduced. The reduction in the thickness of the disk is caused from the following reason.
If an optical system including a short-wave laser unit and having a high-NA is employed, the skew margin for the disk is reduced. The skew margin is a permissible range in which the writing/reading operation can be performed normally even if the incident angle of a laser beam made from the recording surface of the disk is skewed from a perpendicular direction. The problem of the skew angle arises when the shape and distribution of the laser spot formed on the recording surface are deflected because of the difference in the optical path for the laser beam. Conversely, the difference in the optical path can be reduced (that is, the skew margin can be enlarged) even if the laser beam is skewed (even if the optical axis of the incident laser beam is inclined) by enlarging the thickness of the disk. That is, the reduction in the skew margin occurring when the recording density has been raised can be compensated by the enlargement of the skew margin which can be realized by reducing the thickness of the disk. Therefore, the thickness of the disk has been reduced.
If the reduction in the thickness of the disk is attempted or a disk having a larger radius is employed, the inclination of the magneto-optical disk 1 realized by dint of the load applied from the magnetic head 21 (the slider 22) to the magneto-optical disk 1 is enlarged excessively. Thus, the performance for recording/reproducing a signal deteriorates.
FIG. 3 shows a state in which the magneto-optical disk 1 is deflected. FIG. 3A shows a disk 1X having a relatively small radius and a considerably large thickness. In this case, the disk 1X is free from considerable deflection. FIG. 3B shows a disk 1Y having a relatively large radius and a relatively small thickness. In this case, the disk 1Y is deflected considerably. FIGS. 4A and 4B are enlarged views showing portions shown in FIGS. 3A and 3B in each of which deflection takes place.
If the deflection of the magneto-optical disk 1 is enlarged, the skew angle is affected and deviation takes place between the point to which the laser beams are converged and the area to which a magnetic field is applied.
The influence exerted upon the skew angle will now be described.
An optical pickup 31 for recording/reproducing a signal is, as shown in FIGS. 3A, 3B, 4A and 4B, disposed immediately below the magnetic head 21. If deflection does not take place by dint of the load applied by the slider 22 (or if the amount of the deflection is small), the optical axis J of the laser beam emitted from the optical pickup 31 is made to be substantially perpendicular to the recording surface of the disk, as shown in FIGS. 3A and 4A. That is, the skew angle SKA can be made to be substantially 90.degree..
If the relatively great deflection takes place as shown in FIGS. 3B and 4B and thus the disk is inclined, the skew angle SKA is considerably deviated from 90.degree.. That is, the laser beam cannot impinge on the surface of the disk. In this case, the skew angle exceeds the skew margin, thus causing the performance for recording/reproducing a signal to deteriorate excessively.
The deviation takes place between the light converging position and the area to which the magnetic field is applied. If substantially no deflection takes place, as shown in FIGS. 3A and 4A, the center of the core portion of the magnetic head 21 is brought to a substantially required position with respect to the position (the position of the optical axis J) to which the laser beams emitted from the optical pickup 31 are converged. If relatively great deflection takes place and thus the surface of the disk is inclined, as shown in FIGS. 3B and 4B, the positional relationship between the point to which the laser beams are converged and the center of the core portion of the magnetic head 21 is changed. That is, the magnetic head 21 cannot satisfactorily apply a magnetic field to the position which is being heated by the laser beam. As a result, the recording performance deteriorates critically.